This invention relates to the field of seismic data processing and specifically to methods for detecting arrival events, such as direct arrivals and refractions, and for detecting geometry and statics errors in seismic data.
In seismic prospecting, signals are generated at signal source locations by sources (e.g. air guns, dynamite, and vibrators) which travel through media, sometimes being refracted and reflected, and are received at receivers (e.g. hydrophones and geophones, a.k.a. pressure detectors and particle velocity detectors, respectively). The reflections and refractions include information from which subsurface geology is determined. However, the process of acquiring the data is susceptible to error.
For example, the amplitude of a particular shot from a particular source may not be as designed. A receiver may not be well-coupled (either to the recorder or the ground). Further, problems in sources and receivers may be intermittent. As the number of sources and receivers increase, and as the volume of data increases with more and more dense 3D seismic activities, detection of such error becomes more and more difficult.
Also, it is important to know the precise location of the sources and receivers, relative to one another. Again, as data volume has increased, this has become a particularly difficult problem, especially in marine environments, including ocean bottom cable and towed streamer applications.
Even further, allowance must be made for variation in the elevation of receivers and sources, as well as variance in the depth to the sub-weathered layer, since the perceived depth of a particular event in a record is related to the time difference from the instant of the shot to the reception of the response to the shot. This is typically known as "statics" correction.
In determining actual source or receiver location, and in performing statics corrections, typically, the first energy received by the receiver (a.k.a. the "first break") is used. For example, in some cases, the first break is the "direct arrival" (a signal received from the source which has not been reflected or refracted). In other cases, the first break is a refracted signal (e.g. in some ocean bottom cable situations).
Triangulation or least squares techniques use the first break to determine the relative position of shots and receivers. In the case of statics corrections, a decomposition algorithm is used, having the following general formula: EQU .DELTA.t=.DELTA.xy+shot static error+receiver static error
where .DELTA.xy is the position error of the shot and receiver, and where .DELTA.t is derived from refractors found in the data. See, Yilmaz, Seismic Data Processing, V.2, Ch 3, pp. 155-240, Society of Exploration Geophysicists (Tulsa, 1987).
To determine the correct .DELTA.t function, however, the first break must be determined. This is not a trivial task. In fact, it has proven to be very difficult.
Currently, although attempts at automatic picking of the first break have been made, the picking still requires manual work, for most jobs. This manual work includes the time-consuming visual inspection of the data, since the automated processes are highly sensitive to noise. Further, the manual work may require a priori knowledge of the velocity, which is not always available. Visual inspection for good quality control defeats the very purpose of an automated system. Therefore, a reliable, automated system for detecting seismic events, such as the first break, is needed.
Furthermore, techniques using first breaks can only work with a single event per time axis. And, since the first break is the direct arrival for deep water cases only, traditional methods use only a few direct arrival traces for shallow water, disregarding all other traces where the direct arrival occurs at later time than the first break. A technique in which multiple events, especially one which could distinguish between the direct arrival and differing reflections is needed to provide greater flexibility and accuracy in the geometry correction and statics correction areas.
Even further still, current geometry and statics correction processes use only first break information, making it more difficult to identify the type of error occurring. Therefore, a method is needed for detection and correction of geometry error and a method of detection and correction of statics error which the type of error is identifiable. Even further, current picking algorrythms using correlation and stack/correlation techniques are subject to cycle skips, which is common, but very undesireable.